1. Field of the Invention
The present invention is directed to an apparatus for generating focused shockwaves of the type suitable for use in extracorporeal lithotripsy, and in particular to such an apparatus wherein the shockwaves are electromagnetically generated.
2. Description of the Prior Art
Electromagnetic shockwave sources are known for use for a number of purposes, for example, for non-invasively disintegrating calculi situated in the body of a patient, a therapy known as extracorporeal lithotripsy, or to otherwise non-invasively treat pathological tissue with shockwaves. Such shockwave sources can also be utilized in materials testing, to charge specimens under examination with focused shockwaves. For these purposes, the shockwave source is acoustically coupled to the object which is to be irradiated with shockwaves, so that the generated shockwaves can be conducted into the object. Such electromagnetic shockwave sources generally function by supplying a coil arrangement with a high voltage pulse, as a consequence of which an electrically conductive membrane, disposed close to the coil, is rapidly radially expelled away from the coil, thereby generating a pressure pulse in the form of a cylindrical wave moving away from the membrane in a propagation medium surrounding the membrane. This pressure pulse gradually steepens to form a shockwave. The pressure pulse or shockwave is reflected by a reflecting surface so that the acoustic energy is concentrated at the focus of the reflector, such as at the focus of a paraboloid of revolution. The shockwave source and the object to be treated must be disposed relative to each other so that the region to be irradiated with shockwaves in the object coincides with the focus of the reflector. This constitutes the focal region of the shockwaves.
A shockwave source of the type described above is known as a "large aperture ring-shaped sound source" (LARS). Other types of electromagnetic shockwave sources are known wherein planar shockwaves are initially generated, which are focused with suitable acoustic lenses as described in German 33 28 039, corresponding to U.S. Pat. No. 4,674,505, or wherein a membrane shaped in the form of a portion of a spherical surface is provided, in which case further focusing is not necessary. A device of this latter type is described in German OS34 43295.
In a LARS, by contrast, cylindrical waves are generated, which are then concentrated at a focus by reflection at a reflector having the shape of a paraboloid of revolution. In a known shockwave source of this type, the coil arrangement consists of a plurality of flat coils which are arranged side-by-side in a cylindrically curved surface inside a hollow-cylindrical membrane. The manufacture of this coil arrangement involves rather considerable outlay, and thus results in an expensive device. Moreover, the space available for the coil arrangement is not completely exploited, and thus the efficiency of this known shockwave source is substantially below the theoretically possible efficiency. Additionally, it is not possible to uniformly drive the membrane along its circumference, as a consequence of the fashioning of the coil arrangement. This means that the membrane is subjected to locally differing deformations along its circumference, and is therefor exposed to unfavorable mechanical stresses which can result in a premature failure of the membrane. It has been proposed to provide the membrane with beads at those regions of the membrane coinciding with the regions at which neighboring coils of the coil arrangement adjoin, so that the membrane can be deformed more easily under the action of the driving forces. This measure, however, makes manufacture of the membrane considerably more expensive, and also deteriorates the focusing effect, because an ideal cylindrical wave cannot proceed from a membrane provided with such beads.